Ultraviolet light (UVB) has been demonstrated to induce reactivation of Herpes Simplex Virus in non-genital areas in which prior recurrences had been observed. The purpose of this study is to evaluate the ability of acyclovir to suppress the induction of recurrent Herpes Simplex Virus infection by UVB light. Patients will be randomized on a cycle of radiation to receive oral acyclovir or placebo pills for six days in a double-blind study. Each cycle will consist of four challenge irradiations with four miminal erythematous doses of UVB light each. Each patient will be followed for documentation of recurrent lesions induced by UVB light or spontaneously occuring lesions throughout the course of the study. Acyclovir has been shown to have significant effects on pain, lesion crusting, lesion healing and viral sheading when administered intravenously. It has also been shown to have significant efficacy in relief of pain, itching and viral sheading time when topically administered. Oral as opposed to intravenous or topical acyclovir will be used exclusively in this study. An area of skin which is near but not overlapping sites previously involved with herpetic recurrence will be selected. A template will be utilized to isolate four 1 cm in diameter areas of skin. A special apparatus which can hold four fluorescent tubes at a distance of 25.4 cm from the target site will be utilized. The fluorescent tubes will have output primarily of UVB (290-320 nm) with a reduced output of UVA (320-400 nm) average caucasian skin pigmentation would call for about four minutes exposure with increased melanin requiring more exposure time.